Plotting 3D Surfaces
As far as I know, there isn't an add-on or method that can graph more complex mathematical functions in the way you want. As an alternative, you can put your equation into nodes and then render it with a Volume Scatter shader. 

Begin with the default cube. Add a new material, and add a Texture Coordinate node (Shift + A > Input > Texture Coordinate) and Separate XYZ node (Shift + A > Converter > Separate XYZ):



Then, create your equation using Math nodes (Shift + A > Converter > Math). Here, I created your example equation:



Now, add a Color Mix node (Shift + A > Color > MixRGB) and set it to Multiply. Plug the texture coordinates in to one slot, and a Value node in to the other. This will scale our window:



Finally, multiply the output by 10 to increase the value, then plug it into the Density of a Volume Scatter and a Volume Absorption shader added together:



This will nicely display the function when rendered, though the shading isn't ideal:


click for larger image

To optimize rendering, change the following settings:



Although it doesn't give you a workable mesh, this can definitely give you a renderable graph. 

We can also improve the look of it by coloring it:


click for larger image